This tutorial guides developers through the process of introducing document validation into an existing production MongoDB deployment without impacting users. It covers setting up test data, using MongoDB Compass to reverse engineer the de facto data model and identify anomalies, defining appropriate document validation rules, preventing new documents from being added that don't follow these rules, and bringing existing documents "up to spec" against the new rules. The tutorial demonstrates how to add validation rules to a collection, check for exceptions, clean up legacy documents, and enforce strict mode where any inserts or updates will cause an error if the document doesn't follow the rules.